Sprint has reached a deal to buy T-Mobile US for about US$50 billion, according to news reports on Wednesday.

Sprint, owned by Japan's Softbank, would pay about $40 per share for T-Mobile, The Wall Street Journal reported, citing people familiar with the matter. The deal could still fall apart, the Journal warned.

Such a deal would combine the nation's third- and fourth-largest mobile operators, forming a larger rival to Verizon and AT&T but reducing the U.S. mobile market to just three major national carriers. Because of that change in the competitive lineup, the plan would probably face an uphill battle for regulatory approval.

If regulators rejected the plan, Sprint would have to pay T-Mobile more than $1 billion in cash and other assets, the Journal reported.

Under the proposed terms of the deal, T-Mobile parent company Deutsche Telekom would own 15 percent to 20 percent of the combined company.

Reports of a deal to combine Sprint and T-Mobile have circulated since late last year. Both carriers have struggled against much larger rivals in AT&T and Verizon, and each has only about half as many subscribers as either of the two big players. Just last year, Sprint agreed to sell 80 percent of its shares to Softbank in a deal that gave it a much-needed injection of cash to complete an elaborate network transformation.

However, the U.S. Department of Justice and Federal Communications Commission rejected an earlier attempted buyout of T-Mobile by AT&T, and since then T-Mobile has introduced plans that have helped change the way U.S. carriers sell phones and service. Some federal regulators have indicated they want the U.S. to remain a four-carrier market for just the kind of competitive pressure that an underdog like T-Mobile can put on prices and choice.

Absorbing T-Mobile would also be one more task for a very busy Sprint, which in recent years has been integrating multiple networks while phasing out others.